Wide range alternating currentdirect current voltmeter



June 29, 1954 J. BREWER 2,682,636

WIDE RANGE ALTERNATING CURRENT-DIRECT CURRENT VOLTMETER Filed Aug. 5,1949 2 Sheets-Sheet l 2/ 9 m =r- 1 1 W 53 y 34 A9 /2 3 39 i 3 6 37 I: H03 t 1 J. BREWER June 29, 1954 WIDE RANGE ALTERNATING CURRENT-DIRECTCURRENT VOLTMETER 2. Sheefis-Sheet 2 Filed Aug. 5, 1949 Patented June29, 1954 WIDE RANGE ALTERNATING CURRENT- DIRECT CURRENT VOLTMETER JeromeBrewer, Kansas City, Mo., assignor to Midwest Research Institute, KansasCity, Mo., a corporation of Missouri Application August 5, 1949, SerialNo. 108,829

This invention relates to an improved electrical instrument having awide variety of uses, but one of the principal uses of which is as awide range A. C.-D. C. voltmeter. More particularly, it relates to aninstrument employing one or more glow lamps and which uses either theignition or extinguishing voltage characteristic of that lamp or lampsas an essential indicating element of the circuit.

It is, of course, well known that glow lamps such, for example, as neonglow lamps have a characteristic ignition voltage as well as acharacteristic extinguishing voltage. In the past, advantage has beentaken of the'characteristic of a glow lamp to indicate whether or notthe source of unknown voltage is an alternating source of energy or auni-directional source of energy. More recently, a neon voltmeter hasappeared commercially which utilizes a calibrated potentiometer and iscapable of measuring voltages of an unknown source provided the voltagesof the unknown source are above the ignition voltage of the glow lamp.In other words, by tapping off a portion of the unknown Voltage by meansof a potentiometer, it has been possible to reduce the voltage dropacross the glow lamp until it extinguishes. If such a potentiometer iscalibrated against a known standard, it is possible to provide a readingfor the instrument which will indicate the value of the unknown voltage.A great disadvantage of such an arrangement lies in the fact that it isnot capable of measuring voltages below the ignition voltage of the glowlamp, which voltage in the case of a neon glow lamp lies in the vicinityof 80 volts D. C. or 65 volts A. C. This, of course, very materiallylimits the usefulness of an instrument such as that referred to abovewhich has been available in the past.

One of the principal features and objects of the present invention ispredicated on the concept of connecting in series a second calibratedvoltage source to the unknown voltage source so that the source ofpotential across the indicating glow lamp is derived from two sources,one being the source whose voltage value is desired to be measured. Bysuch an arrangement, it is possible to measure voltages far below theignition voltage of the glow lamp, and thus the usefulness of theelectrical instrument is enormously increased.

.A further object of the present invention is to provide a novel highimpedance A. 0-D. C. voltmeter.

Another object of the present invention is to provide a novel A. (1-D.C. voltmeter employing 1 Claim. (Cl. 32498) 2 a glow lamp which willmeasure either A. C. or D. C. voltages both below and above the ignitionvoltage of the glow lamp.

It has been found that an electrical instrument possessing the abovedescribed and referred to characteristics has a Wide range of usefulnessand with a few additional elements, a multitude of functions. It is,therefore, one of the features and objects of the present invention toprovide an electrical instrument employing one or more glow lamps whichwill function as a relaxation oscillator of variable frequency, whichwill measure resistance and capacitance, which will indicate thepresence or absence of a moderately strong radiation (visible andultra-violet light), which will act as a pulse generator, which will actas a detector of radioactivity and X-rays, and which may be used as aclassroom demonstration instrument to illustrate a Wide variety of usesand purposes of glow lamps.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claim. My inventionitself, however, both as to its organization,

".- manner of construction, and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawings, in which:

Figure 1 is a diagrammatic illustration of an electrical instrumentembodying the novel teachings and principles of the present invention;

Figure 2 is a diagrammatic illustration of a high impedance neonvoltmeter embodying the novel teachings and principles of the presentinvention;

Figure 3 is an isometric view of an electrical instrument embodying myinvention; and

Figure 4 is an isometric view of a demonstrator circuit to illustrateapplications of glow-discharge tubes.

In Figure -l of the drawings, the circuit of an electrical instrument isillustrated which is arr ranged to be connected to two sources ofpotenglow lamp of the electrical instrument is shown as a neon glow lampI4. As previously pointed out, one of the principal features of thepresent invention is predicated on the concept of connecting in seriesthe unknown voltage source or some portion thereof with the knownvoltage source. In the form of the invention shown in Figure 1, theportion of the unknown voltage which is to be added to the known voltageis obtained from a potentiometer I5 and a series connected resistor I6.

More specifically, the potentiometer I5 and the resistor I6 areconnected in series across conductors I and II, while the movablecontact ll of the potentiometer I is connected to one side of the neonglow lamp I4. The known voltage is obtained from a voltage regulatorarrangement which includes connecting a second neon glow lamp toconductors I2 and I3 through resistor elements I9 and 20, respectively.

As is well known to those skilled in the art, this will supply a clippedsine wave of substantially constant peak-to-peak voltage at conductors2I and 22, regardless of voltage variations in the A. C. or D. C.source. A pair of resistors 23 and 2.4 are connected in series betweenconductors 2| and 22. The resistor 23 should preferably be approximatelyof the resistor 24 when the glow lamp is a neon glow lamp. By way ofexample, and not by way of limitation, the resistor 23 may be 100,000ohms, and resistor 24 may be 1 megohm. Likewise, by way of illustration,and not by way of limitation, the resistors I9 and 20 may beapproximately 150,000 ohms apiece, resistor I6 may be approximately50,000 ohms, and the potentiometer I5 may be 500,000 ohms.

The right side of the neon glow lamp I 4 is connected to the mid-pointbetween resistors 23 and 24.

The circuit is preferably grounded on one side as at 25. From the abovedescription, it will at once be apparent that the biasing resistor 24 isin series with the lower portion of the potentiometer I5 and theresistor I6. It will further be apparent that when the lower conductorII of the input is of the same polarity as the upper conductor I 2 ofthe known source, the voltage drop across the biasing resistor 24 isadded to the voltage drop across the low part of the potentiometer I5and the resistor I6 to impress the total voltage on the glow lamp I4.

The voltage derived from the biasing resistor 24 is, of course, belowthe ignition voltage of glow tube I4, because of the voltage drop acrossresistor 23. Since this voltage drop across the biasing resistor 24 isconstant and is maintained constant by the voltage-regulating action ofthe glow lamp I8, it will at once be apparent that by calibrating thepotentiometer I5, the position of the movable contact I! will determinethe amount of the voltage input across conductors I 0 and I I. It willfurther be observed that the instrument with the conductors I2 and I 3connected to the A. C. or D. C. source provides an A. C.D.C. voltmeterwhich will give reasonably accurate readings from approximately 10 voltsto 80 volts.

. Attention is also directed to the fact that by simply disconnectingconductors I2 and I3 from the A. C. or D. C. source, the instrumentwithout any changes whatsoever immediately becomes available to readvoltages from 80 volts up (the practical upper limit under the exampleof values hereinbefore given being in the neighborhood of 1,000 volts).It will, of course, be understood that there will be a second set ofcalibrations for the positions of the movable contact arm IT to coverthis second range.

From the above description, it will be observed that I have provided anextremely simple, inexpensive, practical A. C.-D. C. voltmeter capableof measuring a very wide range of voltages.

The instrument shown in Figure 1 also is provided with means capable ofhaving it act as a relaxation oscillator of variable frequency. To thisend, a capacitor 30 is connected across the glow lamp I4 throughswitches 3I and 32 and conductors 33 and 34, respectively. By way ofexample, and not by way of limitation, the capacitor 30 may be about 0.1microfarad. With the switches 3| and 32 closed, and the conductors I0and I I connected to a D. C. voltage of about volts and the conductorsI2 and I3 not connected, the instrument will act as a relaxationoscillator and will provide the characteristic saw-tooth wave of arelaxation oscillator. By varying the setting of the movable contact armII of the potentiometer I5, the frequency may be varied.

Should it further be desired to analyze or inspect the characteristicshape of the output of the relaxation oscillator, an oscilloscope 3b isconnected across capacitor 30 through switches 36 and 31.

With the circuit illustrated in Figure 1 and described above, and withthe switches 3I and 32 closed and the conductors I0 and I I connected toa source of D. C. voltage of about 100 volts, the instrument may be usedto measure resistance and capacitance, for the frequency of oscillationis inversely proportional to the time constant RC. Thus, the frequencyusing known resistors and capacitors is compared with that obtained withan unknown resistor or capacitor substituted in the circuit. 2

As previously pointed out, the instrument may also be used as anindicator of the presence or absence of a light source. This may beaccomplished by connecting conductors I2 and I3 to -115 volt A. C.source, and by connecting conductors I0 and II to a D. C. source ofabout 30 volts. The potentiometer control I1 is then rotated until theindicator glow lamp I4 is just ready to ignite. If this indicator glowlamp I4 is subjected to a moderately strong light source, theimpingement of the light rays on the glow lamp will cause the glow lampto ignite. The ignition of glow lamp I4 will be indicated by adeflection of a vacuum tube voltmeter (not shown) when connected acrossresistor 24.

Not only will the instrument act as an indicator of the presence orabsence of a moderately strong light source (including ultra-violetlight), but it will also act as a detector of the presence or absence ofmoderately strong gamma radiation, or even X-rays will produce a smallchange in the frequency of oscillation.

The instrument illustrated in Figure 1 may be used for demonstrating thevoltage-regulating properties of a glow lamp by connecting theconductors I2 and I3 to a variable A. C. voltage source and byconnecting an oscilloscope (not shown) to conductors 50 and 5|. If thevoltage applied to the conductors l2 and I3 is varied, thevoltage-regulating properties of the glow lamp may be observed on theoscilloscope, and hence the use of a glow lamp as a peak clipping deviceforproducing alternating voltage pulses can be observed.

The instrument shown in Figure 1 may be used as a pulse generator byconnecting a pulse peaking circuit comprising a capacitor 38 and aresistor? 3-9.;across the glow lamp'll'fl through switches and 4 I andconductorsi I .andi22, respectively. The capacitor 3.8, by vwayhfexample, an'd'notby way of limitation,.-.may-.be approximately 0.00025microfarad and the resistor 39 may be 0.1 megohm. Under suchcircumstances, and when the unit is connected to a 110-115 volt A. C;source through conductors Ifi-and I3,the,output of the glow lamp .I8 isdifferentiated electrically ,and provides at the terminals 42 and 4.3 ofthemesistor '39 f60-cycle pulses possessing steep.-wave fronts of shortduration. These pulses maybe observed by connecting terminals 43 and 42to an oscilloscope (not shown).

From the above description, it will, of course, be appreciated that whenthe instrument is to be used solely as a wide range A. C.-D. C.voltmeter, all of the elements connected across the glow lamps I4 and I8through conductors 33, 34, 52, '53, 54 and 55 may be eliminated.

In Figure 2 of the drawings, I have illustrated a second form of thepresent invention, and more particularly, I have illustrated a highimpedance neon A. C.-D. C. voltmeter which is calibrated when the neonglow lamp ignites as distinct from the first embodiment of the inventionwhich was calibrated when the neon glow lamp extinguishes. In this formof the invention, the output of the voltage regulator is connectedthrough conductors 2I and 22 to the opposite ends of a potentiometer 25.The movable contact 21 of the potentiometer 26 is connected throughconductor I I to one side of the unknown voltage source. The top end ofth potentiometer 2B is connected through a resistor 28 and a pluralityof neon glow lamps 29, 44 and 45 to conductor I0 leading to the otherside of the unknown voltage source. By way of example, and not by way oflimitation, the potentiometer 26 may have a resistance of 500,000 ohms,while the resistor 28 may have a value of 100,000 ohms.

In this form of the invention, it will be apparent from the abovedescription that when the lower conductor II of the unknown voltageinput is of the same polarity at a given time as the upper conductor 2iof the known voltage source, the circuit is adding the proportionalamount of voltage from the known voltage source represented by the upperportion of the potentiometer 26.

The number of neon glow lamps connected in series in the circuit ofFigure 2 will depend upon the voltage range desired. Thus, when avoltage range of approximately -80 volts D. C. or 10-65 volts A. C. isto be determined, only one glow lamp, namely glow lamp 29 will be used.To

this end, glow lamp 44 is shorted out of the circuit by closing a switch46, while glow lamp 45 is shorted out of the circuit by closing theswitch 41. When a higher range is required, switch 46 is opened, butswitch 4! is closed. When a still higher range is desired, both switches46 and 41 will be opened. Additional glow lamps and shorting switchesmay be inserted in series in the circuit without departing from theteachings of the present invention.

In considering the embodiment of the invention illustrated in Figure 2of the drawings, it will be noted that there is substantially no load onthe circuit to be measured, and for that reason, the

6. impedance, it will be'apparent that the instrument illustrated inFigure 2 isahighimpedance voltmeter.

It will furthermore be observed that thepotentiometer 26 may becalibrated'to read voltagedirectly when the neon glow'lamp ignites.

From the above description ofthe embodiments of the inventionillustrated in Figures 1 andi2-of the drawings, itwill be apparent thattheiinstrument-in each "case'may'be used for eithermeasuring A. C.voltages or DC. voltages. When 'AQC. voltages are being measured, theneon glowflamp Moi Figure 1 and the neon glow lamps 29,44 and 45 ofFigure 2 will have both plates glowing, while only one of the plateswill glow when D. C. is being measured. The polarity may, of course, bedetermined by noting which of the two plates of the respectiveindicating glow lamps are glowing.

The simplicity of my invention, especially when it is being used as awide range A. C.-D. C. voltmeter, is exemplified by the isometricview ofthe unit as shown in Figure 3 of the drawings. More specifically. theentire circuit may be conveniently housed within a small box 48 fromwhich the conductors I0 and I I extend and terminate in two contactingprongs 49 and 50. The conductors I2 and I3 also extend from the rear of.the box 48 and terminate in a conventional plug 5I which may beconnected to 110 to 220 A. C. or D. C. or connected to a battery greaterthan volts. The indicator glow lamp I4 and the regulator glow lamp I8each partially extend through openings 52 and 53, respectively, in thetop of the box 48. The movable contact arm I! of the potentiometer I5 isactuated by a knob 54 which carries a pointer 55 overlying a calibrateddisk 56. 4

Figure 4 of the drawings illustrates one manner in which the inventionmay be assembled in a breadboard form and used-as a demonstrator.

The numerals employed in Figure 1 have been re-employed in Figure 4 toidentify similar elements of the circuit. A jack 5! has been employed,however, in the place of the switches 36 and 3! which enables theoscilloscope 35 of Figure 1 to be quickly and conveniently connectedinto the circuit. A jack 58 has been employed to enable the resistor 39and the condenser 38 to be connected across the regulator glow lamp [8or to enable other elements to be connected as desired across theregulator glow lamp I8.

From the above description, it will be apparent that I have provided anextremely simple, low cost electrical instrument and apparatus.

I claim as my invention:

In a method of measuring an unknown voltage with a known voltage source,a glow lamp having a predetermined ignition voltage and an adjustablevoltage divider, the steps of connecting said adjustable voltage dividerto one of said known and unknown voltages to obtain a reference voltagewhich is a fraction of said one of said r voltages, connecting saidreference voltage in series with the other of said unknown and knownvoltages and said glow lamp, and adjusting said voltage divider untilthe difierential between said reference voltage and said other of saidvoltages is equal to said ignition voltage as indicated by said glowlamp, the magnitude of said unknown voltage being then indicated by theposition of said voltage divider.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Number Name Date Bjornson May 20, 1930 5Riszdorfer Sept. 25, 1934 Rentschler Apr. 21, 1936 Stansbury et a1 -1Mar. 7, 1939 McLeod Nov. 18, 1941 Swift Feb. 9, 1943 Keinath June 15,1943 Langer Dec. 19, 1944 Moore Sept. 28, 1948 Number Number 8 Name DateElliott May 30, 1950 Moore Oct. 10, 1950 FOREIGN PATENTS Country DateFrance June 17, 1948 OTHER REFERENCES Publication I-Journa1 of theSociety of Motion Picture Engineers, v01. 28, No. 6, June 1937, pages633-642.

